Molecular Shape and VSEPR Theory - iTeachly.com...Molecular Shape and VSEPR Theory Guided Notes –...

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Molecular Shape and VSEPR Theory Guided Notes – Student Edition

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Polarity of Molecules

Molecules are groups of atoms held together by _________________ bonds. These bonds may be

polar or non-polar. (See lesson 2-2 on covalent bonding for more details on polar and non-polar

bonding)

Polar Molecules

Recall that polar bonds result in one side of the molecule having

a slightly positive electrical charge (_____), while the other has a

slightly negative charge (______). These molecules are called

polar molecules.

The polarity of a molecule depends on:

The molecule containing _________________ bonds

The polar bonds in the molecule being arranged

_________________ around the central atom e.g. water _________ or ammonia

___________. In molecules such as water, the most electronegative atom (in the case of

water, oxygen) pulls the electrons towards itself and away from the other atoms. This makes

the oxygen partially negative due to the presence of the electrons from the hydrogen.

More than one type of atom bonded to a central atom (e.g. trichloromethane CHCl3)

The centers of positive and negative charge not coinciding.

Non-polar Molecules

A non-polar molecule has no overall electrical _________________. There are two types of non-

polar molecules:

1. Molecules with _________________ atoms.

The molecules of elements have identical atoms and therefore have pure covalent bonds.

These bonds are non-polar and so the resulting molecules will also be non-polar. Examples

of non-polar molecules include the gases, _____, ______,______,_______,_______ and the

solids ________ and _________.

However, molecules do not necessarily need to have non-polar bonds exclusively in order to be

considered a non-polar molecule.

Name: ______________________________________________ Period:______ Date:____________



2. Molecules with different atoms (_________________).

Molecules made of different atoms will have polar bonds because the atoms will differ in

their __________________________ (the ability to _________________ bonding

electrons). Molecules with polar bonds can be non-polar if the center of the negative charge

_________________ with the center of the positive charge.

Methane shown to the right is a good example of a non-polar

molecule, with polar bonds. The center of positive charge on each of

the four _________________ ____________ coincides with the

____________ ____________ which is the center of negative charge.

The center of positive (or negative) charge is defined as the point of

_________________ distance from each charge. It is right in the middle

of the positive (or negative charges).

The polar bonds in a molecule may be arranged in such a way that the

overall molecule does not have a positive side and a negative side. If the

polar bonds in a molecule are arranged _________________ around

the central atom, this causes the dipoles to _________________ each

other out. Tetrachloromethane pictured right is a good example of this.

The table below shows some common polar and non-polar molecules.

Substances that contain polar

molecules

Substances that contain non-polar

molecules

Hydrogen chloride HCl Carbon dioxide CO2

Phosphorous trichloride PCl3 Oxygen O2

Ammonia NH3 Hydrogen H2

Water H2O Nitrogen N2

Dichloromethane CH2Cl2 Methane CH4

Name: ______________________________________________ Period:______ Date:____________



Practice problem:

The Lewis structures for two molecules are shown in the table below. Ammonia, NH3 is polar, and

borane BH3 is non-polar. Justify this statement.

Ammonia Borane

Lewis Diagram

Polarity Polar

Non-polar

Justification:

VSEPR Theory and Predicting the Shape of Molecules

Valence shell electron pair repulsion (VSEPR) theory can be used to predict the shape of molecules.

This theory uses the concept of _________________ charges (in this case the _________________

charged electrons) repelling each other. Electrons exist in _________________, also known as

electron sets, spread themselves out around an atom so that there is minimum _________________

between them. This allows the molecules to be at their most _________________. The areas which

electrons occupy are also known as regions of electron density, electron clouds or electron domains.

Name: ______________________________________________ Period:______ Date:____________



The position of the electrons determines how the atoms in a molecule _________________ and

gives the molecule its characteristic _________________. This shape can be predicted from the

___________ _________________ and by counting the number of _________________

_________________ around the central atom.

Establishing the number of electron sets

Electrons in a set cannot be separated without disrupting the bonds. An electron set can be made up

of two _________________ electrons of a lone pair, or the two _________________ electrons of a

single bond, or __________ bonding electrons in a double bond, or ________electrons in a triple

bond.

Once the position of the electron sets has been established around the _________________ atom,

the other atoms are placed in the appropriate corners. The overall shape of the molecule is

determined by the location of the atoms only, as the electron sets are not seen.

Two electron sets

When there are ________ electron sets, there will be a ________bond

angle between the electron sets and the molecule will lie in a

________________________. This arrangement is called

________________.

Three electron sets

When there are three electron sets, the electrons will repel each other so

that they are pushed into the corners of an ________________________.

The bond angle will be ________ with all three sets of electrons lying in the

same plane. This arrangement is called ________________________.

In some cases, one of the three sets may be ________________. These

electrons must also be factored into the electron repulsion and so a similar

bond angle is seen. This arrangement is called _____________________

and has a bond angle of ________.

Name: ______________________________________________ Period:______ Date:____________



Four electron sets

When there are four sets of electrons then they repel each other so that they

are directed into the corners of a ________________. The bond angle will be

________and the electron sets will occupy

________________________________ . This arrangement is called

________________________.

Just as is the case with three electron sets, sometimes one of the four electron

sets is a ________________________. This arrangement is called

________________ ________________and has a bond angle of ________.

Determining Molecular Shape and Polarity

The following steps can be followed to determine the shape of a molecule and the resulting angles

around the central atom.

Example 1: Carbon Dioxide (CO2) Molecule

The electron dot structure of carbon dioxide is seen below:

Step 1:

Draw the Lewis structure for the molecule.

Step 2:

Count the number of electron sets around the central atom to determine the arrangement of the electrons.

Step 3:

Count the number of bonding electron sets. Non-bonding electron pairs are not drawn in the final shape of the diagram.

Name: ______________________________________________ Period:______ Date:____________



There are ________ electron sets around the central carbon atom. Each bonding electron set

represents a double bond or 4 electrons. The two electron sets will repel each other until there is a

much distance between them as possible, making the molecule ________________ in shape and the

bond angle ________. In carbon dioxide, the molecule the slightly negative (________) oxygen

atoms are evenly placed around the slightly positive (+)________atom. The centers of negative

charge and positive charge are both on carbon so the molecule is ________________.

Example Two: Methane (CH4) Molecule

The electron dot (Lewis) structure for methane is seen below:

There are________electron sets around the central carbon atom with each electron set representing

a single bond. Because the four electron sets repel each other, they must be as

far away from each other as possible and are therefore pushed into the corners

of a tetrahedron. So the four ________bonds will point to these corners and the

molecule is called ________________. In the methane molecule, the slightly

positive (+)________________________are evenly spaced around the negative

(-)________________. This means that the center of positive charge and

negative charge are in the same place - on the ________________ atom and the

CH4 molecule has no overall ________________ and is non-polar.

Example Three: Water (H2O)

The electron dot (Lewis) structure for H2O is:

Name: ______________________________________________ Period:______ Date:____________



There are four electron sets around a central ________________ atom. Two of these electron sets

represent ________ bonds and the other two are ________ (non-bonding) pairs. These four

electrons sets will repel each other so that they are as far apart as possible, directing each set into

the corners of a tetrahedron as is seen in methane. However, unlike methane, two of these sets are

lone pairs so instead of seeing the ________bonds of a

tetrahedral, we only see ________ bonds from each of the

hydrogen atoms (________bonds) and the molecule is

therefore ________________________.

The center of positive charge is ________________

between the two hydrogen atoms, while the center of

negative charge is on the ________________atom. This

means that water is a ________________ molecule.

Conventions for drawing 3D wedge and dash structures

When drawing 3D molecules, the following conventions are used:

Example: The Shape of the CF4 molecule

Carbon tetrafluoride (CF4) has ________ sets of bonded electrons. Its Lewis diagram is

shown to the right. Since the molecule is tetrahedral it will show ________ fluorine

atoms in the plane of the page, one pointing ________of the page and one pointing

________the page.

Example: The shape of NH3

Ammonia has ________ sets of electrons; ________ sets are bonding and

________ set is non-bonding. The shape of the molecule is

________________________with one N-H pair in the plane of the page, one

pointing out of the page and one pointing into the page

Name: ______________________________________________ Period:______ Date:____________



Example: The Shape of SO2

Sulfur dioxide (SO2) has ________ sets of bonded electrons and ________ pair

of non-bonded electrons. Since sulfur dioxide is ________________, the bonds

will be ________________________ as the page, as seen in the diagram to the

right.

Practice Problems:

1. Predict the shape, including the bond angle of Trichloramine (NCl3). Use a wedge and dash

diagram to support your answer.

Answer:

2. Predict the shape and bond angle of boron trifluoride (BF3). Draw the Lewis structure to

support your answer.

Answer:

Name: ______________________________________________ Period:______ Date:____________



Testing for Polarity in Liquids

The molecules in liquids are ________________. If they are

placed near an electrically charged object, the

________________molecules will align and be attracted to

the charged object. This is because opposite charges are

________________ one another. The stream of polar liquid

(e.g. water) will be deflected ________________ the charged

object. This provides a test for polarity as

________________liquids will continue to flow past the

charged object ________________ being deflected.

The Polarity of Water Molecules

Water molecules are ________, with polar ________bonds and a ________symmetry. Water has a

higher melting and boiling point than is usual for a substance containing such low mass molecules.

This is due to the fact that there is a relatively large difference in ________________ between the

hydrogen and oxygen atoms in water which produces strong attractive (________________) forces

between the molecules called hydrogen bonds. These bonds require a considerable amount of

________________ to break.

Because water is polar, it can also act as a solvent for some ionic compounds.

Name: ______________________________________________ Period:______ Date:____________



Dissolving

For an ionic solid to dissolve, the strong ________ bonds which hold the ions together must be

broken. The ions are spread among the water molecules. The polar water molecules are attracted to

the ions on the surface of the solid with the ________________being attracted to the

________________ end of the water molecule (i.e. the ________________) and the

________________ attracted to the ________________ end (i.e. the ________________). If the

attraction between the ions and the water is ________________ than the attraction between the

cation and anion, then the solid will dissolve in water. The separated ions are surrounded by water

molecules and form hydrated ions.

The energy required to break apart an ionic lattice (called the ________________), differs for

different ionic compounds. Ionic compounds are considered insoluble in water if ________________

energy is required to separate the ions than can be gained by forming hydrated ions.

Name: ______________________________________________ Period:______ Date:____________



Practice Problem:

Draw an annotated diagram to show how potassium iodide dissolves in water.

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